You would like to have an all-or-nothing phase transition like ice/water. You have drawn its curve. However, you have an extended phase transition in an ensemble of small systems, similar to the liquid/liquid phase transition in micellar surfactant solutions.
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Interestingly, your transition has a large mW/mg jump, and inside it has a small temperature jump. Interesting case. It is necessary to investigate the structure of the alloy in the transition region.